Wave-powered desalination system

Abstract

The system comprises a wave energy converter comprising a moveable body 2 exposed to wave motion coupled to a seawater pump 6 adapted to pump seawater through a reverse osmosis desalination system. An

Description

FIELD OF THE INVENTION This invention relates to a wave-powered desalination system and in particular to a wave-powered desalination system having a control system adapted to optimise the efficiency of the system. BACKGROUND OF THE INVENTION Osmosis is a physical phenomenon by which two liquids, in different concentrations, separated by a semi-permeable membrane, come into contact and tend to even themselves out. The liquid with the lower concentration crosses the membrane toward that with highest concentration. This process is inverted in reverse osmosis and can be used to remove salt from seawater by forcing seawater through a membrane by adding pressure. On the opposite side of the membrane salt-free water is obtained, while the remaining water on the first side of the membrane still holds the salt the membrane prevented from passing through. A wave energy converter (WEC) typically comprises at least one at least partially submerged moveable member adapted to be moved relative to a reaction mass under the action of waves, an energy conversion device, such as a pump, turbine or generator, being mechanically coupled to the moveable member and adapted to convert the kinetic energy of movement of the moveable member relative to the reaction mass due to wave action into useable energy. Therefore a WEC is an ideal power source for a desalination system because a WEC can be located just offshore to use energy from wave motion to pump seawater through the membrane of an adjacent reverse osmosis (RO) desalination system. A number of different wave-powered desalination systems are known. Some known wave powered desalination systems use of the mechanical power captured by a wave energy convertor (WEC) to pump seawater through a reverse osmosis (RO) membrane to produce fresh water. While simple and reliable, a problem with such known systems is that the efficiency of the WEC is affected by the resistive force exerted on the WEC by the RO system. In order to maximise efficiency of the WEC in capturing mechanical power from the waves, the resistive force applied to the WEC by the RO system would need to be varied as a function of the speed of motion of the WEC. However, a typical RO system interfaced directly with a WEC will offer a fixed resistance or damping force (within an operating band) to the WEC motion. The RO membrane is typically specified to perform best with a steady flow of seawater through the membrane. The RO system can be designed to include some smoothing of flow using an accumulator and it has been demonstrated that RO membranes can typically tolerate a degree of intermittency within a fairly broad operating band. Typically, in known wave powered desalination systems a WEC is connected directly to a suitable RO system to pump water through a RO membrane of the RO system at a flow rate and pressure allowing the RO system to operate within its preferred design parameters. However, this negatively impacts the efficiency of the WEC due to the relatively fixed damping / resistive force being applied back through the WEC by the RO system. An object of the present invention is to provide an improved wave-powered desalination system incorporating a mechanism to control a resistive force applied to the WEC by the RO system in order to optimise the performance of the entire wave-powered desalination system. SUMMARY OF THE INVENTION According to the present invention there is provided a wave-powered desalination system comprising a wave energy converter (WEC) comprising a moveable body arranged to be exposed to wave motion such that said wave motion imparts kinetic energy to said movable body, said moveable body being mechanically coupled to a seawater pump adapted to pump sea water through a reverse osmosis membrane of a reverse osmosis (RO) desalination system to thereby desalinate said seawater, wherein an electric motor / generator is arranged to selectively drive or be driven by the mechanical coupling between the moveable body and the seawater pump, a controller being provided for controlling the operation of the electric motor / generator to control the resistance to movement exerted on the moveable body via the mechanical coupling to achieve an optimised linear damping profile. Preferably said controller is adapted to control the operation of the electric motor / generator to respectively, in a first mode of operation, drive the mechanical coupling and, in a second mode of operation, absorb energy from the mechanical coupling, in order to control the resistive force exerted on the moveable body to achieve an optimised linear damping profile required for efficient operation the WEC, while achieving the operating parameters (in terms of water pressure and flow rate from the seawater pump) required by the RO membrane of the RO desalination system. The operation of the electric motor / generator may be controlled by the controller in said first and second modes of operation as a function of the speed of movement of the moveable body. Below a predetermined speed of movement of the moveable member of the WEC, where the resistive force applied by the RO system to the WEC is above an optimal resistive force required by the WEC, the controller may control the motor / generator to operate in said first mode of operation such that a driving force is applied to the mechanical coupling to reduce resistive force exerted on the moveable body of the WEC, and above a predetermined speed of movement of the moveable member of the WEC, where the optimal resistive force to movement of the moveable member required for efficient operation of the WEC is above the resistive force applied to the WEC by the RO system, the controller may control the motor / generator to operate in said second mode of operation such that the motor / generator absorbs energy from the movement of the moveable member of WEC and thereby increases the resistive force experience by the moveable member of the WEC to its optimum level. A battery may be electrically coupled to motor / generator for driving said motor / generator in said first mode of operation and for absorbing energy to thereby charge the battery in said second mode of operation. In said first mode of operation the motor / generator may be powered to increase the driving force applied to the pump, thereby reducing the resistance to movement (resistive force) experience by the moveable member of the WEC. In one embodiment the mechanical coupling may comprise a cable extending between the moveable body and the pump, said cable being arranged to pass around a pulley, said motor / generator being operatively coupled to the pulley. The pump may comprise a piston pump adapted to pump seawater through said RO membrane of the RO system in response to movement of the moveable body. The pump may comprise a pump cylinder coupled to a reaction mass and a piston slideably moveable in said cylinder, said piston being operatively coupled to the moveable body by said mechanical coupling. BRIEF DESCRIPTION OF THE DRAWINGS A wave-powered desalination system in accordance with an embodiment of the present invention will now be described, by way of example only. Figure 1 illustrates the contrasting requirements for optimised operation of a WEC and of an RO membrane of a desalination system; and Figure 2 illustrates a wave-powered desalination system in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS As discussed above, a WEC will operate optimally (i.e. capture most mechanical power from wave action) when the resistive force that the WEC applies to the motion induced by wave action is controlled such that the WEC presents a greater damping to the wave motion (i.e. resistance to movement of a body undergoing displacement by a wave) as the speed of the motion of the body induced by the wave increases. This optimised linear damping profile is illustrated by the solid line in Figure 1. When a WEC is used to pump seawater through a RO membrane of a reverse osmosis (RO) desalination system to perform desalination, a pump of the desalination system will typically present a fixed (typically fixed within a narrow band through each stroke of the pump) damping force to the WEC. This is illustrated by the broken line in Figure 1. In a wave-powered desalination system in accordance with an embodiment of the present invention, an electric motor / generator is introduced into the power train of the RO desalination system and is controlled such that the resistive force experienced / seen by the WEC achieves the desired optimised linear damping profile require by the WEC whilst allowing the RO desalination system to operate within its optimised operating parameters in respect of water flow and water pressure applied to the RO membrane of the RO desalination system... A wave-powered desalination system in accordance with an embodiment of the present invention is illustrated in Figure 2. A moving body 2, typically a buoyant member, is arranged for reciprocal movement under the action of waves with respect to a reaction mass 4. A piston pump 6 is arranged between the body 2 and reaction mass 4 and coupled to a fluid circuit of the RO desalination system via suitable check valves 7 such that motion of the body 2 under the action of waves operates the pump 6 to pump seawater through a membrane of the RO desalination system. A spring 8 may be provided to provide a return biasing force on the piston pump 6. This arrangement enables the kinetic energy produced by the WEC under the action of waves on the moving body 2 to be utilised to pump sea water through the membrane of the RO desalination system. In the embodiment shown in Figure 2, the mechanical coupling between the body 2 of the WEC and the piston pump 6 may comprise a cable, a portion of which is wrapped around a pulley 10, whereby the reciprocating motion of the body 2 is converted to a rotary motion of the pulley 10. However, other types of mechanical coupling are envisaged. The pulley is coupled to an electric motor / generator 12. The motor / generator 12 is preferably electrically coupled to a battery 14. A controller is provided to control the operation of the electric motor / generator 12 to respectively drive the pulley 10 or absorb energy from the rotation of the pulley 10 (by acting as a generator to charge the battery) in order to control the resistive force exerted on the moving body 2 to achieve the desired optimised linear damping profile required by the WEC, while achieving the operating parameters (in terms of water pressure and flow rate) required by the RO membrane of the RO desalination system. For example, at lower speeds of movement of the moveable member 2, where the resistive force applied by the RO system to the WEC (the dotted line in Figure 1) is above the optimal resistive force (solid line in Figure 1), a driving force would be applied to ensure the optimal resistive force is achieved. The can be achieved by using the battery 14 to drive the motor / generator 12 and increase the driving force applied to the pump 6, thereby reducing the resistance to movement (resistive force) experience by the moveable member 2 of the WEC. At higher speeds of movement of the moveable member 2, where the optimal resistive force (solid line in Figure 1) to movement of the moveable member 2 required for efficient operation of the WEC is above the resistive force applied to the WEC by the RO system (dotted line in Figure 1), the motor / generator 12 may be operated as a generator, absorbing energy from the movement of the moveable member 2 of WEC and increasing the resistive force experience by the moveable member 2 of the WEC to its optimum level. In this mode, the ‘braking’ action of the motor / generator 12 generates electricity which is stored in the battery 14. This would be achieved in a similar manner to the process used for ‘regenerative braking’ in electric vehicles and electrically powered cycles. An alternative energy source, such as solar panels or wind turbines, may be coupled to the battery 14 to charge the battery 14 and ensure sufficient electrical power is available for driving the motor / generator 12 when required. Therefore, the motor / generator 12 is controlled to present an optimal resistive force to the movement of the moveable member 2 of the WEC by ‘braking’ or ‘driving’ the mechanical pump 6 in a manner that will result in the optimal restive force profile for the WEC (i.e. the solid line in Figure 2 for the example shown) while maintaining the optimal operating parameters required for the RO system. While expending energy to increase the resistive force exerted against the motion of the mechanical piston pump 6 may seem counter-intuitive, because it will reduce the flow of water to the RO membrane of the RO system during that specific stroke, it will maximise fresh water production because it has the potential to maximise the mechanical power captured by the WEC and therefore increase fresh water production over an extended period of time. Incorporating a motor / generator that can be used to apply additional resistive force to the motion of the moveable body 2 of the WEC has an additional benefit related to increasing the operating envelop of the WEC. WEC systems will typically have an operating limit in terms of sea-state beyond which the WEC will not be able to operate. Adding an ability to apply additional resistive force to the moveable body of the WEC will extend the point at which the WEC’s operating limit is exceeded and allow the overall system to continue operating as a desalination system in a wider range of sea-states, therefore producing more fresh water. The invention is not limited to the embodiment described herein but can be amended or modified without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A wave-powered desalination system comprising a wave energy converter (WEC) comprising a moveable body arranged to be exposed to wave motion such that said wave motion imparts kinetic energy to said movable body, said moveable body being mechanically coupled to a seawater pump adapted to pump sea water through a reverse osmosis membrane of a reverse osmosis (RO) desalination system to thereby desalinate said seawater, wherein an electric motor / generator is arranged to selectively drive or be driven by the mechanical coupling between the moveable body and the seawater pump, a controller being provided for controlling the operation of the electric motor / generator to control the resistance to movement exerted on the moveable body via the mechanical coupling to achieve an optimised linear damping profile.

2. A system as claimed in claim 1, wherein said controller is adapted to control the operation of the electric motor / generator to respectively, in a first mode of operation, drive the mechanical coupling and, in a second mode of operation, absorb energy from the mechanical coupling, in order to control the resistive force exerted on the moveable body to achieve an optimised linear damping profile required for efficient operation the WEC, while achieving the operating parameters (in terms of water pressure and flow rate from the seawater pump) required by the RO membrane of the RO desalination system.

3. A system as claimed in claim 2, wherein operation of the electric motor / generator is controlled by the controller in said first and second modes of operation as a function of the speed of movement of the moveable body.

4. A system as claimed in claim 3, wherein below a predetermined speed of movement of the moveable member of the WEC, where the resistive force applied by the RO system to the WEC is above an optimal resistive force required by the WEC, the controller controls the motor / generator to operate in said first mode of operation such that a driving force is applied to the mechanical coupling to reduce resistive force exerted on the moveable body of the WEC, and above a predetermined speed of movement of the moveable member of the WEC, where theoptimal resistive force to movement of the moveable member required for efficient operation of the WEC is above the resistive force applied to the WEC by the RO system, the controller controls the motor / generator to operate is said second mode of operation such that the motor / generator absorbs energy from the movement of the moveable member of WEC and thereby increases the resistive force experience by the moveable member of the WEC to its optimum level.

5. A system as claimed in claim 4, wherein a battery is electrically coupled to motor / generator for driving said motor / generator in said first mode of operation and for absorbing energy to thereby charge the battery in said second mode of operation.

6. A system as claimed in any of claims 2 to 5, wherein, in said first mode of operation the motor / generator is powered to increase the driving force applied to the pump, thereby reducing the resistance to movement (resistive force) experience by the moveable member of the WEC.

7. A system as claimed in any preceding claim, wherein the mechanical coupling comprises a cable extending between the moveable body and the pump, said cable being arranged to pass around a pulley, said motor / generator being operatively coupled to the pulley.

8. A system as claimed in any preceding claim, wherein said pump comprises a piston pump adapted to pump seawater through said RO membrane of the RO system in response to movement of the moveable body.

9. A system as claimed in claim 8, wherein said pump comprises a pump cylinder coupled to a reaction mass and a piston slideably moveable in said cylinder, said piston being operatively coupled to the moveable body by said mechanical coupling.

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